Is it possible in AWS XRay to create the sampling rule somehow that will sample all the calls for some service with duration greater than some value?
The only way right now to find the lagging sub-service is to sample 100% and then filter by service name and duration. This is pretty expensive having 10K+ segments per second.
AWS X-Ray dev here. Biased sampling on longer duration can skew your service graph statistics and cause false negatives. If you are ok with this data skew, depend on which X-Ray SDK you are using you might be able to achieve conditional sampling by making explicit sampling decisions on segment close. This would require you to override some certain part of the SDK behaviors.
We know this is a popular feature request and we are working on improving it. Please use our AWS X-Ray public forum https://forums.aws.amazon.com/forum.jspa?forumID=241&start=0 for latest feature launch or provide any additional comments.