Look at the file var/export_performance.json.log and make a graph out of the query time to see if it went down.

Notes:

• The most interesting value in times is donations.query because that's the pure query time, not related to the number of donations exported.
• There are two entries for each day, one for the "preparation run" of the export and the other for the "real" run of the export. Another interesting statistic would be how much they differ, if the 2nd one is always faster or slower.

Example jq query:
jq '{export_date, query_time: .times["donations.query"]}' export_performance.json.log | less