INSERT takes over 5 hours, when inserting more than specific number of rows
My guess would be - if you are not actually being blocked - that you are hitting a threshold above which the data file (and/or log file) has to grow, and that your configuration is not optimized to support this growth. Ensure that:


Your data file growth rate is reasonable, and is a fixed size (not %!) large enough to accommodate this transaction and anything else that may be going on concurrently.
Same for log file.
Instant file initialization is enabled. This will help speed up data file growths but not the often more troublesome log file growths.
You aren't inserting 1.35 million individual rows in a loop or otherwise all as one big transaction. There is something to be said for breaking your transactions into chunks.