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LMDB use of sparse or non-sparse data file

I had to duplicate an LMDB database for replication recently, and used mdb_copy to do so.
One server is using the original data.mdb database (which is sparse) and the other is using the mdb_copy non-sparse data.mdb file. 
The two servers are identical (hardware, OS, software and configuration). OpenLDAP-2.4.39 is being used, 64 bit Linux OS. mdb_stat shows the map size as the same, which is expected.

Will the use of the non-sparse file cause any performance issues?

The reason for asking is that I am seeing a difference in search times between the two.
With 20 million objects, a search on modifyTimestamp (which is indexed) gives:
server 1: approx 1s
server 2: approx 60s

server 2 started with the same search time as server 1 when the databases were originally copied, but has slowly increased its search time over about a week for this same search.