The bug

When a minecart travels along a corner rail, normally it experiences no speed loss. When a block is placed next to the track in the direction that the minecart was originally traveling before turning, the minecart experiences a massive speed decrease. This can be seen by having a diagonal track with blocks on either side of the track.

Contradictory to this, when a minecart travels along a straight track with blocks beside the track, no slowdown is evident. This issue is solely present with corner tracks.

This appears to be an issue with the minecart's hitbox 'rubbing' against the block before fully turning, resulting in a loss of speed when it should not occur. The minecart hitbox needs to be adjusted so that it remains within the confines of its block while it turns.

This issue can be best seen in the following video, created in Beta 1.5. Here, the bug is explained in further detail, including the importance of where the block is in relation to the direction of the turn.

Video demonstrating the issue: https://www.youtube.com/watch?v=9jMpB_wwce4

A fix for this was provided by user thinkofdeath in the comments of issue MC-51053, which has the same underlying problem.
https://bugs.mojang.com/browse/MC-51053?focusedCommentId=223854&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-223854