Full record view with mra fleshing causes an internal timeout

Evergreen 2.12.9 (fork)
OpenSRF 2.5.2
Ubuntu Xenial 16.04 lts
PostGresql 9.5

This week we upgraded from 2.7.1 to 2.12.9.

For records that share sometimes thousands of similar titles (which is typical for our visual record descriptions) our staff noticed a timeout for a full view with the staff client and web browser.

The view is initiated with the WWW Record.pm module, where information on relates records is gathered, as shown here in red at line 92 and 228:
https://github.com/IISH/Evergreen/commit/c256f6efd3f482e8a07b02f118291874629282da

In the backend this translates into a request-response similar to this srfsh CLI:
request open-ils.cstore open-ils.cstore.json_query {"from":["unapi.mmr","984156","marcxml","record","{holdings_xml,mra,mmr.unapi}","NL-AMISG","0","acn=>5,acp=>5,bre=>5",null,null,null]}

Received no data from server

------------------------------------
Request Time in seconds: 16.894166
------------------------------------

The log:

srfsh 2018-02-21 15:12:01 [DEBG:10483:osrf_stack.c:115:1519222068104830] Session [1519222304.591612.151922230410483] found or built
srfsh 2018-02-21 15:12:01 [DEBG:10483:osrf_stack.c:123:1519222068104830] We received 1 messages from <email address hidden>/open-ils.cstore_drone_n-013_1519202283.708662_5219
srfsh 2018-02-21 15:12:01 [DEBG:10483:osrf_stack.c:254:1519222068104830] passing client message 1 / session 1519222304.591612.151922230410483 to app handler
srfsh 2018-02-21 15:12:01 [DEBG:10483:osrf_app_session.c:125:1519222068104830] received partial message response
srfsh 2018-02-21 15:12:01 [DEBG:10483:osrf_app_session.c:137:1519222068104830] adding 50735 bytes to response buffer
srfsh 2018-02-21 15:12:01 [INFO:10483:osrf_stack.c:163:1519222068104830] Message processing duration 0.000093
srfsh 2018-02-21 15:12:01 [DEBG:10483:osrf_stack.c:166:1519222068104830] after msg delete
srfsh 2018-02-21 15:12:01 [INFO:10483:osrf_app_session.c:394:1519222068104830] Returning NULL from app_request_recv after timeout: open-ils.cstore.json_query [{"from":["unapi.mmr","984156","marcxml","record","{holdings_xml,mra,mmr.unapi}","NL-AMISG","0","acn=>5,acp=>5,bre=>5",null,null,null]}]
srfsh 2018-02-21 15:12:01 [DEBG:10483:osrf_app_session.c:1249:1519222068104830] AppSession [open-ils.cstore] [1519222304.591612.151922230410483] destroying self and deleting requests

Whereby the apache web requests would wait apparently forever for a response it will never receive.

To work around this, I removed the mra field from the call.

We expect the list with the mra would either be limited if too large... or possibly disabled with a configuration option.