This is a patch fix support to address the Redis driver timeout issue on your Sitecore 9.1 CD instance to address the lack of threads to respond to the blocking request whilst waiting to complete the response callback from Redis service.
Refer to Sitecore KB Article under Sitecore 9.1 Update 1:
Sitecore Support Patch:
A few other optimisations were performed such as adding the pollingMaxExpiredSessionsPerSecond=”100″ attribute to the redis sessionstate provider and adding an applicationInitialization section to boost the startup time of the Sitecore CD Application.
Result: Redis timeout exceptions are no longer appearing in the logs.
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