High CPU usage / load average while no running queries

Each request/query ends in these GC logs:

2019-01-02T14:33:52.738+0100: 93297.860: Total time for which application threads were stopped: 0.1159665 seconds, Stopping threads took: 0.0469178 seconds
2019-01-02T14:33:54.286+0100: 93299.408: Total time for which application threads were stopped: 0.0003418 seconds, Stopping threads took: 0.0000811 seconds
2019-01-02T14:34:14.733+0100: 93319.855: Total time for which application threads were stopped: 0.0016874 seconds, Stopping threads took: 0.0013020 seconds
2019-01-02T14:34:14.734+0100: 93319.856: Total time for which application threads were stopped: 0.0001006 seconds, Stopping threads took: 0.0000226 seconds
2019-01-02T14:34:14.945+0100: 93320.068: Total time for which application threads were stopped: 0.0004190 seconds, Stopping threads took: 0.0002370 seconds
2019-01-02T14:34:14.953+0100: 93320.075: Total time for which application threads were stopped: 0.0051206 seconds, Stopping threads took: 0.0014569 seconds
2019-01-02T14:34:15.987+0100: 93321.110: Total time for which application threads were stopped: 0.0137072 seconds, Stopping threads took: 0.0134333 seconds
2019-01-02T14:34:16.999+0100: 93322.121: Total time for which application threads were stopped: 0.0105055 seconds, Stopping threads took: 0.0089762 seconds
2019-01-02T14:34:19.053+0100: 93324.175: Total time for which application threads were stopped: 0.0539037 seconds, Stopping threads took: 0.0526524 seconds
2019-01-02T14:34:20.036+0100: 93325.158: Total time for which application threads were stopped: 0.0178477 seconds, Stopping threads took: 0.0167766 seconds
2019-01-02T14:34:20.363+0100: 93325.485: Total time for which application threads were stopped: 0.0002115 seconds, Stopping threads took: 0.0000587 seconds
2019-01-02T14:34:45.088+0100: 93350.210: Total time for which application threads were stopped: 0.0004575 seconds, Stopping threads took: 0.0001255 seconds
2019-01-02T14:34:45.089+0100: 93350.211: Total time for which application threads were stopped: 0.0001138 seconds, Stopping threads took: 0.0000229 seconds
2019-01-02T14:34:46.115+0100: 93351.237: Total time for which application threads were stopped: 0.0254978 seconds, Stopping threads took: 0.0252771 seconds
2019-01-02T14:34:46.201+0100: 93351.323: Total time for which application threads were stopped: 0.0109330 seconds, Stopping threads took: 0.0107341 seconds
2019-01-02T14:34:46.246+0100: 93351.368: Total time for which application threads were stopped: 0.0396406 seconds, Stopping threads took: 0.0394395 seconds
2019-01-02T14:34:46.717+0100: 93351.839: Total time for which application threads were stopped: 0.0076333 seconds, Stopping threads took: 0.0073539 seconds
2019-01-02T14:34:49.480+0100: 93354.602: Total time for which application threads were stopped: 0.0098339 seconds, Stopping threads took: 0.0040309 seconds
2019-01-02T14:35:49.870+0100: 93414.997: Total time for which application threads were stopped: 0.6677784 seconds, Stopping threads took: 0.1241825 seconds
2019-01-02T14:35:50.076+0100: 93415.198: Total time for which application threads were stopped: 0.0014716 seconds, Stopping threads took: 0.0000543 seconds
2019-01-02T14:35:54.090+0100: 93419.212: Total time for which application threads were stopped: 0.0125148 seconds, Stopping threads took: 0.0092182 seconds
2019-01-02T14:35:55.091+0100: 93420.213: Total time for which application threads were stopped: 0.0007178 seconds, Stopping threads took: 0.0005540 seconds
2019-01-02T14:35:56.093+0100: 93421.216: Total time for which application threads were stopped: 0.0011982 seconds, Stopping threads took: 0.0000786 seconds
2019-01-02T14:36:01.104+0100: 93426.226: Total time for which application threads were stopped: 0.0094146 seconds, Stopping threads took: 0.0005936 seconds
2019-01-02T14:36:01.920+0100: 93427.042: Total time for which application threads were stopped: 0.0003037 seconds, Stopping threads took: 0.0000605 seconds
2019-01-02T14:36:50.955+0100: 93476.078: Total time for which application threads were stopped: 0.0259146 seconds, Stopping threads took: 0.0156997 seconds
2019-01-02T14:36:56.042+0100: 93481.165: Total time for which application threads were stopped: 0.0287458 seconds, Stopping threads took: 0.0210005 seconds
2019-01-02T14:36:57.803+0100: 93482.925: Total time for which application threads were stopped: 0.0209145 seconds, Stopping threads took: 0.0040108 seconds
2019-01-02T14:36:57.819+0100: 93482.942: Total time for which application threads were stopped: 0.0043863 seconds, Stopping threads took: 0.0042021 seconds
2019-01-02T14:37:01.132+0100: 93486.254: [GC (Allocation Failure) 2019-01-02T14:37:01.139+0100: 93486.261: [ParNew
Desired survivor size 8716288 bytes, new threshold 6 (max 6)
- age   1:     542824 bytes,     542824 total
- age   2:       3744 bytes,     546568 total
- age   3:       2848 bytes,     549416 total
- age   4:        624 bytes,     550040 total
- age   5:        440 bytes,     550480 total
- age   6:       4576 bytes,     555056 total
: 136793K->772K(153344K), 7.9592690 secs] 287927K->152081K(1031552K), 7.9669336 secs] [Times: user=4.82 sys=2.76, real=7.96 secs]

And sometimes I get this one in the middle of the others:

2019-01-02T14:37:01.132+0100: 93486.254: [GC (Allocation Failure) 2019-01-02T14:37:01.139+0100: 93486.261: [ParNew
Desired survivor size 8716288 bytes, new threshold 6 (max 6)
- age   1:     542824 bytes,     542824 total
- age   2:       3744 bytes,     546568 total
- age   3:       2848 bytes,     549416 total
- age   4:        624 bytes,     550040 total
- age   5:        440 bytes,     550480 total
- age   6:       4576 bytes,     555056 total
: 136793K->772K(153344K), 7.9592690 secs] 287927K->152081K(1031552K), 7.9669336 secs] [Times: user=4.82 sys=2.76, real=7.96 secs]

Queries may last up to 10 seconds, where 30-500ms was enough before this problem.