Increased latency after switching to Aurora I/O optimized

We recently switched our database from using the simple RDS PostgreSQL to Aurora I/O Optimized. For our use-case, it turns out that this did not lead to noticeable cost savings, but at least we now have a more resilient setup, running in a cluster, and this makes it trivial to add more read replicas in the future, if and when the need arises. The problem is, though, that the latency of our queries went way up. We had the previous setup tuned quite well, and our average latency was well below 100ms. After switching, the average shot up to maybe 1000ms. This is a bit puzzling, as Aurora I/O Optimized is also advertised as having better performance than the other options. When looking at the performance insights, the only obvious difference is that the read replica spends a lot of time on IO:SLRURead, which was not there at all on the old setup. I've tried searching around for more information on this, but came up pretty much empty handed. The question, I guess is: how can I figure out what the cause of this degraded performance might be and what parameters could I maybe tune in order to attempt to improve things?