In our project we have been using QGIS 1.8 Lisboa with a MySQL database placed on a server accessed via local network since two years. Everything is working fine and quick. The vector layers can be edited very swiftly, even when the MySQL server is on internet.
Since the change to QGIS 2.0 this is no more the case (the same applies to 2.2, 2.4, 2.6 and 2.8). Even a simple vector layer of 10 features takes a long time to open. It is not a question of rendering. The response time is extremely long even when opening the attribute table values). Naturally, the more features there are, the longer it takes. We were actually forced to continue using 1.8 for editing and we use 2.6 only for printing. It should be noted that we are using 1.8 and 2.6 as parallel installations on the same computers now, and the difference in speed is enormous (c. 1-3 seconds response time for a simple table on QGIS 1.8 and c. 20 seconds on QGIS 2.6). We have computers running Windows 7, Windows 8, Linux Ubuntu 14.04. The operating system has no effect on the speed.
Switching between two versions is very cumbersome for our workflow (but still much quicker than working with 2.6 only).
Actualization (30th April 2015): The problem is not related with network. I have made additional tests with localhost, and the results show the same pattern. The old 1.8 version is immediate in its response time, while the 2+ versions take a very long response time. I have however observed an additional, highly remarkable fact - the speed of the QGIS 2.0-2.8 versions is related to the number of tables in the MySQL database!!! If there is only a single table (with 34 simple polygon records), opening its attribute table in QGIS 2.6.1 takes 2-3 seconds. It is not immediate like QGIS 1.8, but it is pretty quick. Now, if I leave this simple polygon layer opened in QGIS 2.6.1 and I add another tables with spatial information into the MySQL database (yet I do not load them in QGIS, they remain only present in the database), the time necessary to open the attribute table of that simple polygon layer will increase (even if the polygon layer is still the only one layer loaded in QGIS)!!! With 4 tables it takes 5 seconds to open the attribute table, with 9 tables it takes 10 seconds, with 81 tables it takes more than 70 seconds to open an attribute table of a single loaded layer. I just do not understand why this happens only by QGIS 2 and higher??? QGIS 1.8 is in no way influenced by the number of MySQL tables. Can be related with GDAL-tools??? I cannot believe we are the only project that has this problem. Can someone replicate this QGIS/MySQL behaviour? Our configuration of MySQL: version 5.6.21, the tables use MyISAM database engine.