Hi @sns ,
You can check for warnings and errors in the crawl log to check if the file exceeds the search limit, which can affect performance.
We could also go to Central Administration> Search Service Application> in the Diagnostics section, click Crawl Health Reports to check Crawl Rate,Crawl Latency, CPU and Memory Load and so on.
To improve crawl performance, you can do the following:
Slow response time from crawled servers >>Provide more CPU and RAM and faster disk I/O
Low network bandwidth >>Install one or two one-gigabit-per-second network adapters on each crawl server
Content processing >>Provide more content processing components, and more CPU resources for each content processing component
Slow processing by the index components >>Add I/O resources for servers that host index components
If you want to crawl only one site collection, create a new content source, choose SharePoint Sites type, select "only crawl the site collection of each start address", then type the site collection url in start address(first remove this site from other content source), then run full crawl for the new content source.
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