Msecs per row jumps 6x at about 10,000? (no replies)

I have a php page that reads a mysql table (an event log) and outputs the results in an html table. There can be a lot of text in some rows so I am not expecting super-fast loading.

There are 17,000 records in the dataset, and I really wanted to display them all at one view. But that times out at 120 seconds.

So I experimented at reported time (time measured at top of php page, and at bottom, listing the difference) vs number of rows served. The graph is at winster.org/wamp.jpg

The graph show a roughly-linear section up to about 10,000 rows, and a second roughly-linear section above 10,000 rows - but at a much steeper rate: the first 10,000 rows take approx 0.4 msecs per row; beyond 10,000 rows, it takes about 2.5 msecs per row. Times for the first 8000 and the last 8,000 rows are pretty much the same, so it doesn't look like a rogue record - it seems to be a sudden slowing at about the 10,000 record mark.

Uninstalling 32-bit WAMP and moving to 64-bit made load times SLOWER by about 10%. Increasing key_buffer_size to 2G and max_allowed_packet to 256M made no difference (in fact about 1% slower)

Any suggestions of why this might be happening?

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Win 64 Pro 64-bit running on i5 with 8Gb RAM. Wamp 3.0 32-bit (and later 64-bit) with Apache=2.4.23, Mysql=5.7.14, PHP=5.6.25
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