We're having increasing problems with our MySQL slave lagging, and much of it seems to be the constant barrage of lastactivity updates to the user database. Worse, they sometimes get stuck for a few full seconds:
| 2 | system user | | dbname | Connect | 58 | end | UPDATE user
SET lastactivity = 1357163120
WHERE userid = 80109 |
We're running at ~100,000 total users. We've usually got 1,000-1,500 on simultaneously.

We've added an index to lastactivity.

Any suggestions for better resolution on this?

- - - Updated - - -

For further reference:
mysql> show index from user;
+-------+------------+-----------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-------+------------+-----------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+
| user | 0 | PRIMARY | 1 | userid | A | 112256 | NULL | NULL | | BTREE | |
| user | 1 | usergroupid | 1 | usergroupid | A | 4 | NULL | NULL | | BTREE | |
| user | 1 | username | 1 | username | A | 112256 | NULL | NULL | | BTREE | |
| user | 1 | birthday_search | 1 | birthday_search | A | 8635 | NULL | NULL | | BTREE | |
| user | 1 | birthday | 1 | birthday | A | 37418 | NULL | NULL | | BTREE | |
| user | 1 | birthday | 2 | showbirthday | A | 56128 | NULL | NULL | | BTREE | |
| user | 1 | referrerid | 1 | referrerid | A | 14032 | NULL | NULL | | BTREE | |
| user | 1 | fbuserid | 1 | fbuserid | A | 4 | NULL | NULL | | BTREE | |
| user | 1 | email | 1 | email | A | 112256 | NULL | NULL | | BTREE | |
| user | 1 | lastactivity | 1 | lastactivity | A | 112256 | NULL | NULL | | BTREE | |
+-------+------------+-----------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+
10 rows in set (0.00 sec)