No it's not. From the offical [Only registered and activated users can see links. ]: 'Speed Class and UHS Speed Class symbols indicate *minimum writing performance to ensure smooth writing of streaming content such as video shooting. This is important mainly for camcorders, video recorders and other devices with video recording capabilities. *Speed Class designates minimum writing performance to record video. The Speed Classes defined by the SD Association are Class 2, 4, 6 and 10.' [*Emphasis added]
I didn't say 'class 4 is always 4MB/sec', and anyway that's minimum writing performance. What i said was 'The class (speed) of the card only indicates its write speed, the read speed is the same, class 4-10. As the unit usually does not write to the card but only reads from it, the class is of no importance on the unit. But it sure helps when you are writing large maps to it.' and I stand by that.
As stated, the Class (writing speed) is important in things like camcorders that are WRITING large amounts of data to the cards. Garmin units are not normally writing to the card, just reading from it. In further expanation, of 'The class (speed) of the card only indicates its write speed', this is in regard to the relative write speed i.e. Class 4 is faster than Class 2, 6 is faster than 4, & 10 is faster than 6.
The above is also what i have found anacdotally.
Flash memory slows in relation to the the total quantity of data that has been written/overwritten onto it, so a lot of reformats and write-cycles will eventually compromise its read speed i suppose. Old cards are maybe slower because the 'wear levelling' capabilities are reduced. That relative read slowness effect you have experienced may be simply more to do with the relative write usage of the cards, rather than as you are thinking due to their class (which is the relative write speed only).
Sorry Loly, i think you have misunderstood me.
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