Time difference for a query by changing one field?

[Goerge Meier]

Hello,
i do two querys overa database with 1.000.000 entries.
One is:
SELECT `id`, `Name`, `mtime`, `lat`, `lon`,ROUND(Speed,0) as Speed, `note` FROM DBLoc JOIN ( SELECT IMEI as i, Max(mtime) as d FROM DBLoc WHERE mtime >= date_sub(now(), interval 36 hour) OR Speed= -10 GROUP BY Name) AS QUERY2 ON Name= i AND mtime = d LIMIT 100

This one needs 2.3 seconds.

The same query but `mtime` ist changed to `date` (nearly same column style)
Same table etc. And this only takes 0.52 seconds.

Anybody an odea WHY this could be faster?
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