This occured on Test Wikipedia — https://test.wikipedia.org/wiki/Special:Block/Nov-1-2018-test — on November 1, 2018. I didn't test for blocks of 9, 8, 7, or 6 pages but I imagine they are also time consuming.
This video shows 4 work flows: https://drive.google.com/open?id=1Msu6Hvgc3X7yEg-DkuXMItIlGbBnwGry
Use case 1: Setting a large partial block
0:00 – Start setting partial block with 10 pages for user with no current block
0:18 — Click “block this user”
0:19 — “Block succeeded” displays
OUTCOME: ✅ Success, acceptable user experience.
PROCESSING TIME: 1 second
Use case 2: Modifying a large partial block, adding and removing
00:26 — begin modifying block to remove one page from 10 and add an alternate
00:35 — click “re-block the user with these settings”
1:29 — “Database error” page displays
OUTCOME: ❌ Failure, unacceptable user experience
PROCESSING TIME: 54 seconds
[W9s60gpAIC8AABuS6FQAAAAM] /wiki/Special:Block/Nov-1-2018-test Wikimedia\Rdbms\DBTransactionSizeError from line 1354 of /srv/mediawiki/php-1.33.0-wmf.2/includes/libs/rdbms/loadbalancer/LoadBalancer.php: Transaction spent 8.6079864501953 second(s) in writes, exceeding the limit of 3. Backtrace: #0 /srv/mediawiki/php-1.33.0-wmf.2/includes/libs/rdbms/loadbalancer/LoadBalancer.php(1776): Closure$Wikimedia\Rdbms\LoadBalancer::approveMasterChanges(Wikimedia\Rdbms\DatabaseMysqli) #1 /srv/mediawiki/php-1.33.0-wmf.2/includes/libs/rdbms/loadbalancer/LoadBalancer.php(1364): Wikimedia\Rdbms\LoadBalancer->forEachOpenMasterConnection(Closure$Wikimedia\Rdbms\LoadBalancer::approveMasterChanges;1858) #2 /srv/mediawiki/php-1.33.0-wmf.2/includes/libs/rdbms/lbfactory/LBFactory.php(219): Wikimedia\Rdbms\LoadBalancer->approveMasterChanges(array) #3 /srv/mediawiki/php-1.33.0-wmf.2/includes/libs/rdbms/lbfactory/LBFactoryMulti.php(413): Closure$Wikimedia\Rdbms\LBFactory::forEachLBCallMethod(Wikimedia\Rdbms\LoadBalancer, string, array) #4 /srv/mediawiki/php-1.33.0-wmf.2/includes/libs/rdbms/lbfactory/LBFactory.php(222): Wikimedia\Rdbms\LBFactoryMulti->forEachLB(Closure$Wikimedia\Rdbms\LBFactory::forEachLBCallMethod;1660, array) #5 /srv/mediawiki/php-1.33.0-wmf.2/includes/libs/rdbms/lbfactory/LBFactory.php(270): Wikimedia\Rdbms\LBFactory->forEachLBCallMethod(string, array) #6 /srv/mediawiki/php-1.33.0-wmf.2/includes/MediaWiki.php(595): Wikimedia\Rdbms\LBFactory->commitMasterChanges(string, array) #7 /srv/mediawiki/php-1.33.0-wmf.2/includes/MediaWiki.php(568): MediaWiki::preOutputCommit(RequestContext, Closure$MediaWiki::main;1825) #8 /srv/mediawiki/php-1.33.0-wmf.2/includes/MediaWiki.php(878): MediaWiki->doPreOutputCommit(Closure$MediaWiki::main;1825) #9 /srv/mediawiki/php-1.33.0-wmf.2/includes/MediaWiki.php(517): MediaWiki->main() #10 /srv/mediawiki/php-1.33.0-wmf.2/index.php(42): MediaWiki->run() #11 /srv/mediawiki/w/index.php(3): include(string) #12 {main}
Use case 3: Modifying a large partial block, removing
1:40 — begin modifying block to remove one page from 10
1:45 — click “re-block the user with these settings”
2:38 — “block succeeded” displays
OUTCOME: ❌ Success, but unacceptable user experience
PROCESSING TIME: 53 seconds
Use case 3: Modifying a large partial block, adding
2:48 — begin modifying block to add one page to 9
2:54 — click “re-block the user with these settings”
2:59 — “block succeeded” displays
OUTCOME: ✅ Success, acceptable user experience.
PROCESSING TIME: 5 seconds